In grooved-end mechanical pipe joining systems, pipes and pipe fittings are joined by a collar that engages grooves in the ends of the pipes and pipe fittings. This type of mechanical joining system is commonly used, for example, in piping for automatic fire protection systems. FIG. 1 is a drawing of a pipe elbow 5 for grooved-end mechanical joining as may be formed by a casting process, as further described below. The pipe elbow 5 includes a central curved portion 6 that defines an elbow bend and tangential end portions, or tangents 7, 8, extending linearly from the ends of the curved portion 6. The tangents 7, 8 serve as the fastening joints. To fit in a system, the elbow 5 is formed within geometrical constraints including a specified distance S from the axial centerline of one tangent end to the end face of the opposite tangent end. In addition, a tangent axial length T and a diameter D of the elbow are chosen so that the elbow can mate with pipes in piping system.
The tangents 7, 8 are each provided with a circumferential groove 9, 10. In a connecting pipe, a groove is also provided. FIG. 2 shows in part section a cast elbow joint or fitting 11 attached to a grooved pipe end 12. When the fitting 11 is butted against the pipe 12, the groove 16 on the pipe elbow is parallel to the groove 18 on the pipe end. A fastening collar 14 engages the grooves 16, 18 in the abutting ends of the elbow and pipe to fasten the elbow 11 and pipe 12.
Referring again to FIG. 1, the inner radius R1 bend of the conventional pipe elbow 5 presents difficulties for manufacturing, particularly in light wall pipe as is used in fire protection systems. To form a smooth curve, the radius R1 is typically taken from a center of curvature C1 determined by the intersection of the inner margins 7a, 8a of the tangents 7, 8 as illustrated in FIG. 1. Bending the pipe to form the central curved portion does not form the tangents, however, which must be shaped in a separate step. When the pipe is further worked to form the tangents, for example in a die, the work typically creates unwanted distortion in the inner radius wall. If the radius is too small, buckling of the inner radius occurs. If the radius is too large, bulging occurs.
As a result of these problems, elbows fittings for grooved-end systems are usually produced as a casting or by welding together pipe segments to avoid the bending-related problems.
Casting techniques, however, are limited to forming parts that typically have a minimum wall thickness that exceeds the thickness of light-wall pipes used in fire protection systems. FIG. 2 illustrates a cast pipe fitting 11 connected to a light wall pipe 12, in which the difference in wall thickness between the fitting 11 and the pipe 12, and the resulting difference in internal diameters, may be seen. The change in flow passage area caused by a cast pipe fitting in a system adds flow resistance and pumping losses to the system, which increases the installation and operating costs of such systems.
A fabricated elbow fitting 20, shown in FIG. 3, is welded from straight pipe segments. The welded fitting 20 is formed with a groove 22 that is connected by a collar 14 to the pipe. Because of the shape transitions and irregular corners, a weld fabricated elbow introduces even greater flow restrictions and pumping losses in a pipe system than a cast fitting.
In addition, both casting and weld fabrication elbows are costly to produce, and add expense to the piping system.